In general, a base station or BTS (Base Transceiver Station) antenna for mobile communication is placed in a high position such as the top of a building, and electric waves emitted from the antenna is received by mobile communication terminals on the ground.
Such a BTS antenna is provided with directivity so that mobile communication terminals on the ground receive electric waves at the same reception or input level regardless of their locations.
The BTS antenna forms a beam, e.g., cosecant squared beam (without a null in a depression angle range of up to 45 degrees from the horizontal plane) in the elevation plane, to cause substantially uniform input electric field on the ground in a predetermined depression angle range.
FIG. 1 is a diagram showing the construction of a conventional cosecant squared beam antenna. In the cosecant squared beam antenna, antenna elements are arrayed vertically, and hereinafter a description will be made on the assumption that antenna elements are arrayed vertically. In this construction, a beam emitted from each antenna element is formed with flares to achieve such directivity that electromagnetic waves are radiated within a predetermined angle in the horizontal plane.
Besides, a plurality of the antenna elements are arranged in a vertical linear array to form a beam in the vertical direction. The amplitudes of the antenna elements 2 or the upper half of the array and the antenna elements 3 or the lower half of the array are symmetrical about the center (e.g., the amplitude of the top antenna element is the same as that of the bottom one). The phases of all the antenna elements 2 are identical. Similarly, the phases of all the antenna elements 3 are identical. The phase of the antenna elements 2 is shifted with respect to that of the antenna elements 3 by a prescribed amount.
With this construction, the antenna radiation pattern assumes a cosecant squared pattern in the vertical plane, resulting in substantially uniform input level in a range of depression angle from the horizontal plane.
However, if a beam is formed in this manner, as shown in FIG. 2, in an area at a depression angle over 45 degrees from the horizontal plane with respect to the BTS antenna, i.e., around the foot of the antenna, the input level is necessarily reduced.
FIG. 3 is a diagram showing the phase characteristics of the conventional cosecant squared beam antenna. The phase characteristics indicates the relation between angles and phases in the vertical plane at points equally distant from the origin as an observation point at the center of the array.
Referring to FIG. 3, in an area lower than the horizontal plane or in an area at a depression angle of 0 (zero) degrees or more, the phase is at 0 degrees. On the other hand, in an area at a depression angle less than 0 degrees or in an area at an elevation angle, the phase is at 180 degrees at almost all angles. This means that, with the horizontal plane as a boundary face or an interface, electromagnetic waves radiated to below the horizontal plane and those radiated to above the horizontal plane are in phase opposition.
FIG. 4 is a diagram showing the radiation or directivity characteristics of the conventional cosecant squared beam antenna in the vertical plane. In FIG. 4, in an area at a depression angle of 45 degrees or more, the radiation characteristics deteriorate. That is, an area in the vicinity of the antenna, at a depression angle of not less than 45 degrees, involves a null.
In Japanese Patent Application laid open No. HEI9-246859, there has been disclosed “Antenna” as a conventional technique for improving the radiation characteristics in the vicinity of the antenna. In the conventional technique, an array antenna consists of a first antenna element with wide directivity in the zenith direction and second antenna elements with narrow directivity in a direction at a prescribed angle from the zenith direction, which are arranged around the first antenna element. Thus, the input level of mobile terminals is maintained constant.
However, the conventional technique is aimed at reducing nulls caused in the direction of the front of the antenna for a campus base station. Therefore, if the technique is applied to a base station for mobile communication, the gain of the antenna is significantly reduced in the direction at a depression angle of 90 degrees.
As just described, there has not been proposed a wide-angle null-fill antenna preventing a null or the presence of an insensitive area in the direction at a depression angle of 90 degrees.